The present invention relates to a method of operating an internal combustion engine, in particular of a motor vehicle.
More particularly, it relates to a method of operating an internal combustion engine, in which fuel is supplied by at least one supply pump with a cyclically varying supply output in a pressure accumulator, and injected from it under pressure by at least one injection valve at an injection time directly into a combustion chamber of the internal combustion engine, wherein the pressure acting on the fuel is measured. It is in particular switched to the phase of start operation of the internal combustion engine.
The present invention also relates to a corresponding control device, in particular for such an internal combustion engine.
The above described method is known in particular for motor vehicles with direct-injection diesel or gasoline motors. In them an injection valve is associated with each combustion chamber, with which the fuel is injected under pressure into the corresponding combustion chamber. For producing the pressure acting on the fuel, a supply pump is provided which pumps the fuel to the injection valves. Before the corresponding injection, the fuel is supplied however first to a so-called pressure storage, with which the combustion chamber or the combustion chambers of the internal combustion engine spacially communicate through one of several injection valves. The fuel pressure required for the direct injection is built up in the pressure storage by the supply pump.
With the gasoline direct injection the fuel pressure is increasingly important, since it is decisively responsible for the quality of the preparation and the penetration depth of the fuel in the combustion chamber. In particular in the so-called xe2x80x9cshift operationxe2x80x9d in contrast to the homogenous operation, it is necessary that the fuel at certain time and location is supplied definitely into the combustion chamber. In order to use the total potential of the pressure which is basically available during the direct-injection combustion, different pressures of the fuel during the injection in the combustion chamber are provided, depending on the operation time of combustion.
For measuring the fuel mass to be injected in the combustion chamber, the pressure which acts on the fuel during the corresponding injection is important, so for example for the same fuel mass to be injected at a high pressure, only a short injection time is required, while to the contrary with a low press re the corresponding injection valve must be controlled longer to be in its open condition.
A corresponding injection device is disclosed for example in the German patent document DE 43 11 738 A1. With this device the injection pressure measured by a pressure sensor, together with variables which are characteristic of the operational condition of the internal combustion engine, are supplied as further variables to an electronic control device for determination of the required opening time of the injection valve.
In the internal combustion engines which are known from the prior art, first an electrical pre-supply pump is provided, which produces a pressure of substantially 4 bar, depending on the motor rotary speed. The pre-supply press ure is then lifted by a main supply pump which is driven mechanically directly by the internal combustion engine to a high pressure of substantially 40-120 bar. The supply power or the supply pressure of the main supply pump is substantially dependent on the motor rotary speed and the number of the pistons of the pump.
In addition, up to now in the internal combustion engines operating in a start operation, the main supply pump is not used first for a pressure increase. Moreover, only a pre-supply pressure is built up by means of a valve, via a corresponding control of the valve for a control device. The reason for this procedure is that in the start operation, the pressure as a function of the motor rotary speed, injected fuel quantity, injection time, etc can not be adjusted in advance between the pre-supply and substantially 120 bar, and thereby the quantity of the injected fuel is not calculatable.
Furthermore, in the above mentioned direct-injection motors the injection time is very limited, since the injection can be carried out with a closed outlet valve of the combustion chamber and in a time period, in which the cylinder pressure is smaller than the pressure of the pressure storage.
In order to provide, in addition to the pressure generation, also a controlled regulation of the fuel pressure, it is further known to determine the pressure which acts in the pressure storage on the fuel by means of a pressure sensor. With the knowledge of this pressure, it is then possible by controlling an injection valve, for example by opening of the valve over a predetermined time interval, to perform the injection process in a controlled manner.
With regard to the spacial design of the pressure storage, there is a demand based on technical reasons, to increase the storage volumes. For example it is known that a temperature-dependent formation of gas bubbles in fuel in the (spacial) proximity to the injection valves can be efficiently counteracted by an increase of the pressure storage volume. In contrast, there is however a tendency, especially from cost reasons, to reduce the structural dimensions and thereby the output of the main supply pump. As a result, the time required for the pressure build up in the pressure storage during the start operation is rather increased than reduced.
The above mentioned problem becomes even worse in a main supply pump which is designed as a single cylinder pump, in that in particular with low rotary speeds which occur for example during the phase of starting of the internal combustion engine, the supply stream available from the pump is subjected to strong fluctuations in time, or in some cases to substantially periodic fluctuations.
It is further known that exactly during cold start, emissions can leave the combustion chamber almost unfiltered. In contrast to the EURO2-exhaust norms whose provisions deal with the end of a cold start, the future EURO3-and EURO4 norms also take into consideration (anticipated) the start emissions.
Accordingly, it is an object of present invention to provide a method for operating an internal combustion engine, as well as an internal combustion engine, in which the above mentioned disadvantages are eliminated.
In particular, with the inventive method and the internal combustion engine, the use of a motor-driven main supply pump with a smallest supply output is possible despite the rotary speed fluctuations occurring in the phase of the start operation and thereby the accompanied pressure fluctuations. This generally takes place in the situations, when rotary speed fluctuations or low rotary speeds lead to corresponding pressure fluctuations with regard to the supply pressure of a fuel supply pump.
In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in a method in which the cycles of the supply output of the supply pump and the injection time of the fuel are determined in time relative to one another.
This objective in the inventive control device is achieved in that, the means for in-time determination of the cyclically changing supply output of the supply pump and injection time are provided.
The main concept of the invention is that during a fuel supply of a gasoline direct-injection internal combustion engine the supply stream as well as the pressure build up of the fuel in the pressure storage can be performed in a time range, in which in particular during a cold start, fuel is injected into the combustion chamber.
The inventive method also sets a scenario, in which the fuel pressure provided by a supply pump is varied over time, preferably in a pulsating or cyclically changing manner, so that the fuel pressure which is built up in a pressure storage or is already set is varied over time. These variations can be based for example on the supply pump which is undersupplied from the drive at low rotary speeds, since the supply pump guarantees the required supply quantity of the fuel only from a predetermined rotary speed, or in the starting phase of the operation of the internal combustion engine, when low rotary speeds of the internal combustion engine take place and moreover the maximal pressure required in the pressure storage is located in the building up phase and the buffer action of the pressure storage which makes possible a constant pressure did not come into action.
In accordance with a first embodiment, it can be provided that the aspiration/compression stroke of the supply pump is adjusted to the phase position of the internal combustion engine. It is thereby automatically guaranteed that the supply pump operates in the stroke of the internal combustion engine and thereby the injection times which strictly correlate in time with the phase length of the internal combustion engine are determined with regard to the pumping strokes.
In an alternative embodiment of the inventive method, it can be provided that the aspiration/compression stroke of the supply pump can be determined by drive cams which are suitably arranged on a cam shaft of the internal combustion engine. In this embodiment it is not required to provide a determination of the stroke control of the supply pump to the working stroke of the internal combustion engine by a control unit. Moreover, it is automatically guaranteed that the supply pump, independently from further influences is strictly determined to the machine cycle of the internal combustion engine and thereby no adjustment or tuning of the strike can be performed.
In the case of the start operation of the internal combustion engine, in particular during cold start, it can be further provided that the fuel in the region of maxima of the fuel pressure is injected into the combustion chamber. Directly during the cold start it is necessary to inject relatively great quantities of fuel into the combustion chamber, or in other words the internal combustion chamber must be supplied with at least a rich fuel mixture. Based on the injection correspondingly in the region of fuel pressure maxima it is thereby guaranteed that in each case the maximum possible fuel quantity is available during the cold start since the quantity or mass of the fuel injected in the combustion chamber otherwise is controllable only by the geometry of the injection valve opening or a change of the opening time of the injection valve.
In the inventive method it can be further provided that the value of the fuel pressure measured over a time period can be scanned, that in the region of a pressure maxima of the fuel the injection starts, that the total mass of the fuel injected at an injection time is determined by summing or integration of the product of pressure in the corresponding time interval in a corresponding time interval, and that the injection valve or valves after reaching a fuel mass suitable for the present operational condition of the internal combustion engine is or are closed. Thereby, by suitable selection of the supply time and a quantity integral over the product pressure x time in a preferable manner a relatively great fuel quantity can be introduced in the combustion chamber. In correspondence with the inventive idea exactly this pressure increase is used so that, by the calculation of the above mentioned quantity integral, the fuel is supplied in optimal quantity to the internal combustion engine.
The internal combustion engine in accordance with the present invention is provided for this solution with the corresponding means for determination of the cycles of the supply build up of the supply pump which is varied in time, to the injection times.